Re-chucking. What am I doing wrong?!

[Michael Beswick]

Relative newbie.....Practice bowl on which to use my newly re-ground B-O-B. Ash about 6 x 3".
Initially fitted on to a screw chuck (the type that fit in to the chuck jaws), I created the outer shape. As it's more accessible, I finished the outer and base (to 400), and cut a recess for the chuck jaws. However once re-mounted on the chuck jaws it's clear the bowl is running fractionally out of true. It's not wildly out, and I can re-true it. But then it's back to finishing it again.

 

[Odie]

Michael....there are two things to consider here. When you transitioned from the screw chuck to the recess for mounting the bowl, you are almost never going to get a perfect match from one method of mounting, to the other.....and, it will almost always run un-true to the first mounting. Further.....If you had any catches, or overpressure issues while on the screw chuck, the bowl will tighten further on the screw. The result of that will likely be the bowl will not be exactly true to the screw chuck anymore.

-----odie-----

 

[Dean Center]

As you know, when you make a recess or tenon, you have to cut it carefully, with a flat surface for the chuck jaws to meet and the right angle to the side walls for the jaws in use. I've been really impressed with how much a little saw dust can booger things up. I use a tenon, so the troublesome saw dust can be laying inside the jaws, and if you use a recess, it might be lingering inside the recess. Just one stray splinter can make a huge impact. Sometimes the wood has moved a little, as it dries further or the internal stresses are relieved, as you turn the outside, and this can make the 'flat' at the bottom of the recess un-flat.

In any case, lots of possible sources, some remediable, some not. If you're going to turn something with thin walls or a natural edge, the untrueness will show up as variation in wall thickness around the rim, and you'll want to retrue the bowl when you turn it around. Which makes getting a nice surface and sanding it wasted effort, I'm afraid. For thicker walled utility pieces, it doesn't make as noticeable or important a difference. With Odie's precision work, this would be totally intolerable and he's probably as sensitive to it as anyone, so heed his advice.

 

[Randy Anderson]

I've had this issue when doing plates and platters where precision is critical for details. I hardly ever use a recess for mounting but have found the type of jaw can matter as well as the size of your recess. There's a sweet spot for where jaws make a circle rather than an oval. If you can get your recess the size of the circle of the jaws you'll get a better consistent fit. My nova jaws are more dovetail shaped and do well on a recess of the right size. My Oneway jaws work better on tenons for me but can be very inconsistent for remounting due to the ridges inside the jaws. Not usually a problem for almost all of what I do but I can tell the difference. As Dean points out, a small chip or non flat surface inside your recess can make a big difference. I watch Richard Raffan remount things that spin true all the time with ease. I'm not there yet....

 

[hockenbery]

A couple things that will help recentering
i cut the recess with a spindle gouge and match the jaw profile. always leaves clean surfaces.
It you use a scraper sharpen before you scrape the recess and inspect the recess be sure there are no raised fibers in it.
cut the recess to match the perfect circle of the jaws. (Jaws open with an 1/8” gap will be close to the perfect circle)

one final note. When I’m finish turning a bowl with a thick rim 3/8” of 1/2” if the remounting is off center less than a 1/16 it can’t be felt or seen so I just hollow the bowl. On a thin rim i usually finish turn it in the chuck.

 

[Darryl Fective]

For best accuracy, fit your tenon or mortise to your chuck jaws. The jaws are only a round circle when the there is just a small gap between them, so a mortise should be made so the fully closed jaws just barely fit inside. for a tenon, it should be only slightly larger than the almost closed jaws. That creates the maximum contact area of wood to jaws and distorts (or marks) the piece the least. If you have a tenon way larger, when you tighten the chuck, it will be just the corners of the jaws gripping. They dig into the wood and some will go deeper than others and throw the work off-center.

 

[Michael Beswick]

Thanks to all. About the only thing I am confident about was the recess size for a perfect circle of the jaws. I usually use a tenon but decided a recess was more appropriate this time . Cutting it was not as simple as usual for some reason. The project was over a few days so the piece could have moved. I shall re true it and continue

 

[robo hippy]

I do have a video on You Tube, Mounting things on the lathe. I do cover a bunch of things there. For me, I use a recess almost always. one secret for me is to use a NRS (negative rake scraper) for the final cut where the chuck jaws will mount, or a shear scrape. There always seems to be a little 'bounce' from the end grain/side grain pattern that you go through in standard bowl turning. A NRS or a shear scrape do not rub the bevel, so you can get a slightly more even cut and no bounce. If you spin the bowl by hand and are 1/16 out of round, that means you are plus/minus 1/32 of an inch. Not anything to worry about unless you are trying to turn a 1/16 wall thickness. Most of my bowls are about 1/4 inch wall thickness.

robo hippy

 

[Michael Beswick]

I was tempted to put a dial gauge on it but ultimately decided it was pretty pointless! Truing it up only required a shear scrape so it clearly wasn’t far out. I still need more practice with the bottom bowl gouge. I got a better finish with a NR Scraper at a slight shear angle.

 

[Doug Freeman]

This is why I always wait until the work is reversed and chuck mounted to complete the OD. There are many variables that can effect whether the piece runs true once jaw mounted. I dont want to spend time getting the OD just right (much less the time of sanding/finishing) to then have to redo that work because of a bit of runout.

 

[hockenbery]

Thanks to all. About the only thing I am confident about was the recess size for a perfect circle of the jaws. I usually use a tenon but decided a recess was more appropriate this time . Cutting it was not as simple as usual for some reason. The project was over a few days so the piece could have moved. I shall re true it and continue

when I put a bowl in a Chuck, i true the outside with a pull cut
here are a few gifs I made from a NE bowl from a crotch video. The pull cut almost always cuts the bark cleanly and doesn’t rip it off.


This shows the handle being held against my thigh. Shifting weight and turning my body lets the hand pull the cutting edge of the wing over the surface.

 

[Lennart Delin]

Here is a paper I wrote on how I chuck bowls.